As a gas generating composition for air bags, a non-azide-based gas generating composition comprising a nitrogen-containing organic compound fuel compound serving as a fuel component in combination with an inorganic oxidizing agent, in place of a metal azide compound used so far, has been proposed in recent years. As the non-azide-based gas generating composition, there is a need for a composition generating a large number of moles of gas generated per unit weight to achieve a smaller and lighter gas generator.
Further, the gas generating composition should be safe at the time of production. If the gas generating composition is ignited at the time of production, a large amount of a high-temperature and high-pressure gas is generated, and thus there will be significant physical and human damage. Accordingly, the production process should not involve a step in which the gas generating composition or starting materials can be ignited or exploded.
In recent years, a gas generating composition using ammonium nitrate as an oxidizing agent has been proposed. Ammonium nitrate is ideal as a starting material of a gas generating composition because it generates a large amount of gas and is inexpensive. However, ammonium nitrate is unsatisfactory because it causes a change in density with phase transition in the temperature range where performance required for the gas generating composition is guaranteed. Further, the gas generating composition using ammonium nitrate as an oxidizing agent suffers from a problem that the burning rate is lower than that usually required for the gas generating composition.
To solve these problems, it has been attempted to use ammonium nitrate having a phase stabilized with a potassium salt added to ammonium nitrate. For example, desired ammonium nitrate can be obtained by dissolving ammonium nitrate and a potassium salt in an aqueous solution and then precipitating them.
Further, in an attempt to use ammonium nitrate in a gas generating composition by suppressing the phase transition thereof, WO2000/18704 (and EP0405272, DE3642850, U.S. Pat. No. 5,071,630, etc.) describes preparation of ammonium nitrate having a substantially stabilized phase by forming a complex of ammonium nitrate and a transition metal compound as shown in the following reaction:2NH4NO3+Cu(OH)2→[Cu(NH3)2]2+.(NO3−)2+2H2O
However, the ammonium nitrate having a stabilized phase does still not solve problems such as a low burning rate. By forming an eutectic mixture of the phase-stabilized ammonium nitrate and a fuel component, the ammonium nitrate may be molten at a lower temperature than the melting point of ammonium nitrate and the melting point of the fuel component (hereinafter, referred to as eutectic phenomenon), and for example, a gas generating composition comprising ammonium nitrate having a phase stabilized with potassium nitrate in combination with 5-aminotetrazole excellent in characteristics as a fuel component in the gas generating composition is molten at 108° C., and cannot be used virtually in a gas generator for an air bag in automobiles.
Further, U.S. Pat. No. 6,224,697, 6,143,102, 6,132,538, 6,103,030, 6,039,820, 5,592,812, 5,673,935, and U.S. Pat. No. 5,725,699 describe a gas generating composition using a metal ammine complex approximately corresponding to ammonium nitrate having a phase stabilized by formation of a complex as described above. In particular, U.S. Pat. No. 6,103,030 describes a gas generating composition using a transition metal complex such as diammine copper (II) nitrate and a fuel component such as ammonium nitrate and guanidine nitrate. In these publications, however, it is assumed that a fuel such as guanidine nitrate used conventionally in combination with phase-stabilized ammonium nitrate is used or the fuel component is used as an aid, and it cannot be said that the fuel component can be arbitrarily selected in designing the optimum gas generating composition.
The object of the present invention is to provide an oxidizing agent for gas generating compositions containing ammonium nitrate, which can be safely produced, does not change in density with phase transition, and does not limit usable nitrogen-containing organic compounds serving as fuel components, as well as a gas generating composition with sufficient combustibility using the same.
The present inventors made extensive study to solve the problem described above, and as a result, they found that an oxidizing agent (hereinafter, referred to as heat-treated oxidizing agent) obtained by mixing ammonium nitrate with an inorganic compound having as an element at least one metal atom selected from the group consisting of Cu, Fe, Ni, Zn, Co, Mn and Ti and subjecting the mixture to heat treatment is extremely preferable as an oxidizing agent for gas generating compositions, to complete the present invention.